Soldering tips used to solder PB-Free solder have higher sensitivity to wetting issues caused by oxidation. Sometime the soldering tip surface will become black as below. If this happens, solder will frequently not wet to the surface of the solder tip.

This blackened tip has lost its wettability and cannot supply enough heat to solder components efficiently. This wetting failure may cause:

Carbonized flux and other residues attach onto the plating of the tip.

The plating of the tip is then exposed to air (oxygen) and undergoes high temperature oxidation.

The tin in the solder undergoes high temperature oxidation.

Intermetallic formation between tin and iron begins to consume the tin coating on the tip.

Alloy of tin and iron undergoes high temperature oxidation.

So when using Pb-free solder, some of the below suggestions may help

Do not leave the soldering iron on/hot for long periods of time without making solder connections.

Do not set the tip temperature over 400°C (750ºF).

Use a flux that has an aggressive activation level and long activation window.

Keep the entire tip surface tinned with a coating of solder when not soldering.

If the tip becomes oxidized or blackened and solder will not wet to the tip, you can use chemical paste such as HakkoFS-100, to retin the tip. This kind of chemical paste is specifically designed to remove heavy tip oxides without using corrosive chemicals such as zinc chloride.